Aqueous solutions of sodium hypochlorite arc widely used in cooling water towers; bleaching processes; treatment of recreational waters including swimming pool water, water slide and other water game equipment, spas, and whirlpools; disinfectants; laundry detergents; and, industrial biocides including applications in the petroleum industry. However, a major disadvantage of NaOCl is its instability. As is well known in the art, several methods are used to stabilize NaOCl. The Self et al. reference (U.S. Pat. No. 3,328,294) described a continuous process to stabilize hypochlorite with an equal molar ratio of sulfamic acid. This process was improved upon by Rutkiewic reference (U.S. Pat. No. 3,767,586) who added a buffer which aided in pH control increasing the stability of concentrated solutions.
Bromine has various advantages over chlorine for water treatment such as better performance in high pH or amine environments and a lower volatility. However, sodium hypobromite, the bromine analog to chlorine bleach, is not stable under typical storage conditions, and as such, is not commercially available. Instead, bromine is typically delivered to water treatment systems by various inefficient or inconvenient methods. The art described by either Self et al. or Rutkiewic does not mention a method to stabilize the well known precarious sodium hypobromite molecule as disclosed within this invention. Also, this disclosure shall improve upon the art of Rutkiewic by formulating a more stable, concentrated NaOBr solution in the absence of a buffer.
In one such bromine delivery method, NaBr is oxidized in situ by introducing gaseous chlorine or NaOCl into the process water stream. Another technique uses a stable perbromide (Br.sub.3 --) solution containing 30-40 percent bromine. The perbromide solution releases bromide and bromine when injected into water systems. The formed bromine hydrolyzes instantly to hypobromous and hydrobromic acids. Alternatively, bromine chloride may be added to aqueous process streams wherein it hydrolyzes to hypobromous and hydrochloric acids.
All of these bromine delivery systems have inherit disadvantages. Gaseous chlorine, perbromide, and bromine chloride have high halogen vapor pressures which present safety concerns in handling and storage. Also, these concentrated halogen solutions are corrosive to many metal surfaces found in process equipment either by their high vapor pressures or by the release of one mole of hydrohalic acids in water systems yielding localized low pH environments. As such, none of these methods provide a stable bromine product that can be safely and easily handled while meeting environmental requirements (more fully discussed below), such as low bromate and absorbable organic halogen generation, and having a high free halogen residual and a low volatility (resulting in a greatly reduced odor and vapor-phase corrosion). In addition, a portion of the expensive bromine compound is wasted through an ineffective by-product in some delivery schemes. Thus, the need for a safe, convenient, economical, stable bromine water treatment product remains and is significant.
The Goodenough et al. reference (U.S. Pat. No. 3,558,503), teaches stabilization of bromine using any compound which reacted reversibly with bromine. The disclosed compounds include:
(a) water-soluble primary and secondary amines or amides; and, PA1 (b) sulfamic acid and its water-soluble salts. PA1 a. Mixing an aqueous solution of alkali or alkaline earth metal hypochlorite having from about 5 percent to about 70 percent available halogen as chlorine with a water soluble bromide ion source; PA1 b. Allowing the bromide ion source and the alkali or alkaline earth metal hypochlorite to react to form a 0.5 to 70 percent by weight aqueous solution of unstabilized alkali or alkaline earth metal hypobromite; PA1 c. Adding to the unstabilized solution of alkali or alkaline earth metal hypobromite an aqueous solution of an alkali metal sulfamate in a quantity to provide a molar ratio of alkali metal sulfamate to alkali or alkaline earth metal hypobromite is from about 0.5 to about 7; and, PA1 d. Recovering a stabilized aqueous alkali or alkaline earth metal hypobromite solution. PA1 a. Mixing an aqueous solution of alkali or alkaline earth metal hypochlorite having from about 5 percent to about 70 percent available halogen as chlorine with a water soluble bromide ion source; PA1 b. Allowing the bromide ion source and the alkali or alkaline earth metal hypochlorite to react to form a 0.5 to 70 percent by weight aqueous solution of unstabilized alkali or alkaline earth metal hypobromite; PA1 c. Adding to the unstabilized solution of alkali or alkaline earth metal hypobromite an aqueous solution of an alkali metal sulfamate in a quantity to provide a molar ratio of alkali metal sulfamate to alkali or alkaline earth metal hypobromite is from about 0.5 to about 7; and, PA1 d. Recovering a stabilized aqueous alkali or alkaline earth metal hypobromite solution. PA1 a. Mixing an aqueous solution of alkali or alkaline earth metal hypochlorite having from about 5 percent to about 70 percent available halogen as chlorine with a water soluble bromide ion source; PA1 b. Allowing the bromide ion source and the alkali or alkaline earth metal hypochlorite to react to form a 0.5 to 30 percent by weight aqueous solution of unstabilized alkali or alkaline earth metal hypobromite; PA1 c. Adding to the unstabilized solution of alkali or alkaline earth metal hypobromite an aqueous solution of an alkali metal sulfamate in a quantity to provide a molar ratio of alkali metal sulfamate to alkali or alkaline earth metal hypobromite is from about 0.5 to about 7; and, PA1 d. Recovering a stabilized aqueous alkali or alkaline earth metal hypobromite solution. PA1 a. Mixing an aqueous solution of sodium hypochlorite having from about 5 percent to about 30 percent available halogen (as chlorine) with sodium bromide; PA1 b. Allowing the sodium bromide and the sodium hypochlorite to react to form a 0.5 to 30 percent by weight aqueous solution of unstabilized sodium hypobromite; PA1 c. Adding to the unstabilized solution of sodium hypobromite an aqueous solution of an alkali metal sulfamate in a quantity to provide a molar ratio of alkali metal sulfamate to sodium hypobromite of from about 0.5 to about 7; and, PA1 d. Recovering a stabilized aqueous sodium hypobromite solution. PA1 a. Mixing an aqueous solution of alkali or alkaline earth metal hypochlorite [wherein the percent of available halogen (as chlorine) is less than about 5] with a water soluble bromide ion source; PA1 b. Allowing the bromide ion source and the alkali or alkaline earth metal hypochlorite to react to form a 0.5 to 5 percent by weight aqueous solution of unstabilized alkali or alkaline earth metal hypobromite; PA1 c. Adding to the unstabilized solution of alkali or alkaline earth metal hypobromite an aqueous solution of an alkali metal sulfamate having a temperature of at least 50.degree. C. in a quantity to provide a molar ratio of alkali metal sulfamate to alkali or alkaline earth metal hypobromite is from about 0.5 to about 7; and, PA1 d. Recovering a stabilized aqueous alkali or alkaline earth metal hypobromite solution. PA1 a. Mixing an aqueous solution of alkali or alkaline earth metal hypochlorite [wherein the percent of available halogen (as chlorine) is less than about 5] with a water soluble bromide ion source; PA1 b. Allowing the bromide ion source and the alkali or alkaline earth metal hypochlorite to react to form a 0.5 to 5 percent by weight aqueous solution of unstabilized alkali or alkaline earth metal hypobromite; PA1 c. Adding to the unstabilized solution of alkali or alkaline earth metal hypobromite an aqueous solution of an alkali metal sulfamate having a temperature of at least 50.degree. C. in a quantity to provide a molar ratio of alkali metal sulfamate to alkali or alkaline earth metal hypobromite is from about 0.5 to about 7; and, PA1 d. Recovering a stabilized aqueous alkali or alkaline earth metal hypobromite solution. PA1 a. Mixing an aqueous solution of sodium hypochlorite [wherein the percent of available halogen (as chlorine) is less than about 5] with sodium bromide; PA1 b. Allowing the sodium bromide and the sodium hypochlorite to react to form a 0.5 to 5 percent by weight aqueous solution of unstabilized sodium hypobromite; PA1 c. Adding to the unstabilized solution of sodium hypobromite an aqueous solution of an alkali metal sulfamate having a temperature of at least 50.degree. C. in a quantity to provide a molar ratio of alkali metal sulfamate to sodium hypobromite of from about 0.5 to about 7; and, PA1 d. Recovering a stabilized aqueous sodium hypobromite solution.
However, the bromine solutions prepared according to the Goodenough et al. reference teachings are not stable enough for practical use in commercial cooling water, oil field and other industrial applications.
Sulfamic acid, according to the Goodenough et al. reference, is employed as a free acid or as one of its water-soluble salts such as the sodium, potassium or ammonium salt. However, the manner in which the bromine solutions are prepared provide relatively low stabilities and low available halogen concentrations compared with the discoveries claimed within this invention disclosure. The Goodenough et al. reference charges elemental bromine into aqueous solution prior to stabilization. Because elemental bromine is used in the process disclosed in the Goodenough et al. reference, this process is difficult to complete as well as potentially hazardous since elemental bromine is a fuming, corrosive, toxic liquid.
The Goodenough et al. reference mentions that the available bromine concentration immediately following preparation was about 1 percent by weight. The low bromine concentration achieved by this method was due in part to bromine being soluble at just 4 percent in cold water. Additionally, bromine is wasted in the process disclosed in the Goodenough et al. reference. The reaction according to this process is as follows: EQU Br.sub.2 +H.sub.2 O.fwdarw.HOBr+HBr
Because the produced HBr does not function as a biocide, one half of the bromine adds nothing to the strength of the biocidal species, HOBr. This invention disclosure improves on the Goodenough et al. reference by means of a safer, easier, and more economical process.
Much higher levels of available halogen for disinfection were attained using the invention disclosed in this application, as shown in Table I below, by stabilizing the sodium salt (NaOBr) generated during manufacture. As previously mentioned, sodium hypobromite is unstable and therefore not commercially available. If a stabilized form of NaOBr is proposed, the stabilization process must occur quickly after NaOBr is made.
The method described in the Goodenough et al. reference could not achieve these increased bromine levels as the order of reagent addition described in the reference was deemed not critical to the operability of the method. Since NaOBr is synthesized by the following reaction, NaOCl+NaBr.fwdarw.NaOBr+NaCl, addition of the stabilizer prior to bromide oxidation would not permit the formation of NaOBr.
When water is treated with many halogenated biocides, undesirable halogenated organics can be generated as by-products. These compounds are causing increased environmental and health concerns. It is generally known that low molecular weight halogenated organics are more easily biologically degraded than higher molecular weight species. However, the low molecular weight forms may be more toxic to aquatic and mammalian organisms. Differentiation of these halogenated organics is costly, time consuming and requires the use of gas chromatography, high performance liquid chromatography or gel permeation chromatography. Absorbable Organic Halogen, "AOX", was chosen as a method of measuring the sum of halogenated organic compounds without speciation. AOX is used as an effluent monitoring parameter of water or wastewater in Europe and North America. In the United States, the Environmental Protection Agency ("EPA") is looking closely at AOX discharge in the pulp and paper industry. An object of the present invention is to provide a stable NaOBr solution that can be used to control microbial fouling with minimal AOX generation. The problems associated with controlling AOX levels, being a more recent developing environmental concern, have not been previously resolved in the industry.
The United States EPA extrapolates some animal carcinogenesis with the presence of low bromate levels found in drinking water. Bromate may appear from the ozonation of bromide-containing water raising some concerns in the drinking water industry. Bromate may also be formed by the disproportionation of hypobromite. This reaction occurs at a greater rate in alkaline environments. Hence, if bleach is added to a NaBr solution, the high pH environment could lead to the undesirable production of bromate. One use of the present invention, which was previously unknown and is surprising, is to greatly minimize bromate formation by stabilizing hypobromite when conditions are favorable for bromate production.
The petroleum industry experiences biological problems, including microbiologically influenced corrosion, both localized and general, in oil field waters. In addition, bacteria can plug the wellbore surface in waterflood injection wells. The bacteria form slime plugs, reducing injectivity. Treatment with stable bromine water is a convenient method of dealing with these and similar problems.
It is an object of the present invention to provide a process whereby aqueous solutions of sodium hypobromite can be produced which are relatively resistant to degradation and/or decomposition and which are relatively non-corrosive and non-volatile, yet which retain an improved capacity for oxidation and bactericidal activity.
Another object of the present invention is to provide a stable sodium hypobromite solution in which the formation of AOX is minimized while providing improved microbial fouling control. Other objects and advantages of the present invention will become obvious from the following description thereof.